Method for regulating a roasting furnace

ABSTRACT

The invention relates to a method of regulating a roasting furnace in fluidized bed roasting. Part of the roasting furnace grate is separated off into a separate grate section, known as the overflow grate, where the nozzles and the amount of roasting gas blown through them can be regulated independently of the main grate. It is advantageous to position the separately regulated grate in the section of the furnace where the overflow aperture is located.

[0001] The present invention relates to a method of regulating aroasting furnace in fluidized bed roasting. Part of the grate of theroasting furnace is separated into a separate grate section, known as anoverflow grate, where the nozzles and the amount of roasting gas blownthrough them can be regulated independently of the main grate. It isadvantageous to position the separately regulated grate in the sectionof the furnace where the overflow aperture is located.

[0002] The roasting of concentrates such as zinc sulphide concentrateusually takes place using the fluidized bed method. In the roastingprocess, the material to be roasted, a fine-grained concentrate, is fedinto the roasting furnace via the feed units in the wall of the furnaceabove the fluidized bed. On the bottom of the furnace there is a grate,via which oxygen-containing gas is fed in order to fluidize theconcentrate. The grate usually has in the order of 100 gas nozzles/m².As the concentrate becomes fluidized, the height of the feed bed risesto about half that of the fixed material bed. The height of the bed ison average 8-12% of the total height of the furnace. The pressure dropin the furnace is formed by the resistance of the grate and that of thebed. The resistance of the bed is more of less the mass of the bed whenthe bed is in a fluidized state. The pressure drop is in the range of240-280 mbar.

[0003] The concentrate in the fluidized bed is oxidized (burnt) to acalcine by the effect of the oxygen-containing gas fed via the grate,e.g. zinc sulphide concentrate is roasted into zinc oxide. In zincconcentrate roasting the temperature to be used is in the region of900-1050° C. The calcine is partially removed from the furnace throughthe overflow aperture, and partially it travels with the gases to thewaste heat boiler and from there on to the cyclone and electrostaticprecipitators, where the calcine is recovered. In general the overflowaperture is located on the opposite side of the furnace to the feedunits. The calcine removed from the furnace is cooled and ground finelyfor leaching.

[0004] For good roasting it is important to control the bed i.e. the bedshould be good and the fluidizing controlled. Combustion should be ascomplete as possible and the calcine should come out of the furnacewell. The particle size of the calcine is known to be affected by thechemical composition and mineralogy of the concentrate as well as by thetemperature and oxygen enrichment of the roasting gas. Good fluidizingand bed stability can be improved for example by regulating the amountof impurities in the concentrate mixture or by adding water to the fineconcentrate, causing micropelletization. U.S. Pat. No. 5,803,949describes the stabilization of a fluidized bed in zinc concenctrateroasting, where the bed is stabilized by regulating the particle sizedistribution of the bed.

[0005] The actual pressure drop of the roasting furnace is determined bythe particle size and the volume weight of the concentrate in thefluidized bed, the height of the bed in the roasting furnace and thegrate structure. In order for the functioning of the roasting furnace tobe stable, the pressure drop should remain in a certain position in thefurnace. A low pressure drop may be the result of a low bed for example.Thus local hot points may form and sintering may occur.

[0006] Conventionally furnace pressure drop and bed height are regulatedby adding or removing baffle bars located at the lower edge of theoverflow aperture. Pressure drop can also be affected somewhat by theamount of gas fed through the grate, in particular the part caused bythe grate itself. Adding and removing baffle bars may come to the limitand on the other hand, handling the bars themselves is not to berecommended for reasons of industrial hygiene.

[0007] A method has now been developed according to the presentinvention allowing roasting furnace conditions to be regulated, whenmaterial for roasting is fed above the fluidized bed and the fluidizingroasting gas through the grate at the bottom of the roasting furnace,and at least some of the calcined material is removed from the overflowaperture located at the height of the top of the fluidized bed. Part ofthe furnace grate is separated off to form a separate section, known asthe overflow grate, where the nozzles and amount of gas blown throughthem are regulated independently of the main grate. The separatelyregulated grate is located in the same section of the furnace as thecalcine overflow aperture, preferably below the overflow aperture. Theessential features of the invention will become apparent in the attachedpatent claims.

[0008] It has been shown that using a separately regulated grate theratio in which the calcine is removed from the furnace via the overflowaperture/boiler can be regulated. Using an overflow grate can affect theincrease of favourable particle size. It has been found that an overflowgrate can be used to regulate furnace conditions even if there were onlyless than 0.5% of all the nozzles in the grate in its area. The controlrange of the pressure drop of the overflow grate itself shouldpreferably be wide, around 200-2500 mbar.

[0009] In practice it has been noticed that increasing overflow gratepressure drop increases the amount of calcine removed via the overflowaperture in relation to the amount of calcine recovered from elsewhere.On the other hand the capacity of the furnace can also be raised byrouting a larger amount of the calcine via the overflow aperture andthis can be achieved precisely by using the overflow grate. Increasingoverflow grate pressure drop may affect the turbulence of the fluidizedbed, which causes the coarser material in the lower part of the bed torise upwards and to be discharged from the furnace through the overflowaperture.

[0010] The calcine removed from the overflow aperture is cooledpreferably in a vortex cooler. It is known in the prior art that thesulphate content of calcine obtained from a boiler is higher than thatrecovered from a vortex cooler. Calcine containing sulphates can causeblockages in the boiler, so decreasing the amount of calcine obtainedfrom the boiler aids the smooth functioning of the boiler and the wholeprocess.

[0011] The invention is described by the following examples:

EXAMPLE 1

[0012] A production-scale roasting furnace was run with a constantamount of air (42 000 Nm³) and standard baffle bars with a combinedheight of 75 mm. The temperature was held constant at 950° C. and thefeed mixture was also kept constant. It was possible to regulate thefurnace pressure drop by regulating the pressure drop of the overflowgrate as shown in the table below: TABLE 1 Overflow grate pressure dropRoasting furnace pressure drop mbar mbar  500 263 1 000 254 1 200 249

EXAMPLE 2

[0013] A roasting furnace as in example 1 was used. Oxygen (500 Nm³) wasadded to the grate air (44 000 Nm³), whereupon the pressure drop of thefurnace began to rise, but it was stabilized by raising the pressuredrop of the overflow grate from 800 mbar to 1200 mbar.

1. A method for regulating roasting furnace conditions, whereby materialfor roasting is fed above a fluidized bed and a fluidizing roasting gasthrough a grate at the bottom of the furnace, and at least some of thecalcined material is removed from an overflow aperture located at theheight of the top of the fluidized bed, characterized in that part ofthe roasting furnace grate, below the overflow aperture, is separatedoff to form a separate section, an overflow grate, where the gas feedtakes place independently of the main grate.
 2. A method according toclaim 1, characterized in that the amount of nozzles in the overflowgrate is maximum 0.5% of the total nozzles in the grate.
 3. A methodaccording to claim 1, characterized in that the pressure drop of theoverflow grate is regulated within the range of 200-2500 mbar.
 4. Amethod according to claim 1, characterized in that the pressure drop ofthe furnace is adjusted by regulating the pressure drop of the overflowgrate.
 5. A method according to claim 1, characterized in that theamount of calcine to be removed from the overflow aperture is adjustedby regulating the pressure drop of the overflow grate.
 6. A methodaccording to claim 1, characterized in that the particle size of thefluidized bed is adjusted by regulating the pressure drop of theoverflow grate.
 7. A method according to claim 1, characterized in thatthe material to be calcined is a concentrate.
 8. A method according topatent claim 7, characterized in that the material to be calcined is azinc sulphide concentrate.